Why cycles uses so much mem?
The displayed unit is megabyte, therefore you're using approximately 8.4 GB. The value is based on the performed memory allocations and can be higher due than your RAM due to virtual memory and swapping. The operating system can use your hard drive to allocate additional memory, in case all available space on the RAM is already in use. If you're trying to access parts of the virtual memory, that is physically stored on the hard drive, it will cause a page fault and the operating system will load the memory from the hard drive. In order to load this memory, it will have to swap parts of the allocated space from RAM with the allocated space from disk. Swapping is slow since the data rate for a hard disk is lower and latency for I/O is significantly higher than accessing RAM. However the big benefit is that you can use more memory than the physically available RAM.